This invention relates to methods for manufacturing tubes of carbon steels, stainless steels, copper alloys, aluminum alloys and other metals filled with a core of powdery and/or granular substances.
One example of tubes of this type is flux-cored tubular welding electrode wires. Tubular cored electrodes are manufactured by forming a slit steel strip first into a U-shaped section, into which is filled a specified amount of formulated flux supplied from a flux feeder, and then into an O-shaped section. A flux-cored tube made by welding together the edges of the O-shaped section is then drawn through dies to reduce its diameter and passed through an annealing process if necessary. The flux-cored tubular electrode thus prepared is further reduced to the desired diameter and finished as a coiled product.
Welding in the above process is commonly performed by high-frequency welding processes such as high-frequency induction and resistance welding. In these welding processes, the edges of the almost O-shaped section are heated to the melting temperature by applying a high-frequency current and pressed together by a pair of squeeze rolls to produce coalescence.
Cracks sometimes occur in flux-cored tubular electrode wires when their diameter is reduced in the post-welding rolling or drawing operations. This cracking is explained as follows: During welding, some of the oxide and silicate in the flux adheres to the edges of the open tube. At the welding spot, to be more specific, the magnetic field formed by the welding current turns the edges of the open tube into magnetic poles that attract the strongly magnetic or ferromagnetic ingredients of the flux together with some of the weakly magnetic or paramagnetic ones. The attracted substances at the edges fuse into the weld as unwanted inclusions that result in weld defects which, in turn, cause the cracking in the diameter reduction operations. The cracks in the flux-cored tubular electrode wires impair weld quality and welding efficiency.
Usually, compounds of various ingredients selected to meet the intended applications of cored tubes are filled, either as such or in granulated form, in the semi-finished open tubes. The flux-cores of tubular electrode wires, for example, comprise slag formers such as rutile powder and magnesia clinker, arc stabilizers such as sodium silicate and potassium titanate, and deoxidizers and alloying agents such as low-carbon ferrosilicon, ferrosilicon manganese and aluminum magnesium. Iron powder, iron oxides and other ferromagnetic substances are sometimes added for the increase of deposition speed, adjustment of flux packing density, and improvement of welding efficiency. In addition, the flux-cores usually contain not less than 5% iron-bearing ingredients in total, with fine particles ranging from 32-mesh (0.5 mm) to dust accounting for at least 50%. All particles of granulated fluxes and the particles of non-granulated ferroalloys, iron powder and iron oxide contain ferromagnetic iron-bearing ingredients. When preparing or powdering non-iron-bearing materials, contamination with iron powder, iron oxides or other ferromagnetic substances is sometimes unavoidable. Therefore, there is an adequate risk that the magnetized edges of almost closed tubes attract particles of the filled fluxes, not only when they contain iron powder, iron oxides and other ferromagnetic substances but also when they are made of only paramagnetic substances. Particularly, particles whose size is smaller than that at equilibrium where a magnetic attraction working on particles balances with the resisting gravity are high susceptible to the attraction. Furthermore, a certain extent of ingredients segregation is unavoidable in granulated fluxes. Then, the particle size at equilibrium becomes larger when iron segregation occurs concentrically in fine particles, thereby dangerously increasing the number of fine particles susceptible to magnetic attraction and the attraction-to-gravity ratio.
The "Method for Manufacturing Filler Wire" disclosed in Japanese Provisional Patent Publication No. 234792 of 1985 offers a solution for the above problem. The filler wire according to this invention has a core consisting of an upper layer of nonmagnetic material and a lower layer of ferromagnetic or ferritic material, with the upper layer keeping the lower layer from being attracted to the magnetized edges of the almost closed tube. However, the upper layer of only nonmagnetic substances and the lower layer of only ferromagnetic (or ferritic) substances are completely separated from each other. When subjected to stress-relief annealing or dehydrogenating heat treatment during the diameter reduction process, the ferromagnetic substances (such as iron powder) form a coherent mass as a result of sintering. This leads to the localized thinning, and then cracking, of the tube wall during diameter reduction. The "Composite Electrode Wire" disclosed in Japanese Provisional Patent Publication No. 234794 of 1985 has a powdery core of substantially nonmagnetic substances whose specific magnetic permeability is not higher than 1.10 so that the core materials remain unattracted to the magnetized edges of almost closed tube. However, the materials of this core contain no ferromagnetic powder at all. The "Method of Manufacturing Tubes Filled with Powders" disclosed in Japanese Provisional Patent Publication No. 109040 of 1979 leaves a large enough clearance between the edges of an almost closed tube to be welded and the surface of a powdery core contained therein to keep even the stirred powder away from the edges, instead of filling the tube to its utmost limit. However, this method has little effect on the stirring of the powder containing even a very small quantity of ferromagnetic ingredients. The "Method of Manufacturing Composite Tubes" disclosed in Japanese Provisional Patent Publication No. 5897 of 1988 removes finer ingredients than 48-mesh, which can be more readily stirred up than the coarser ones, from a powder fed into the almost closed tube to prevent the adhesion of the finer ingredients to the edges of the tube to be welded. The "Method of Manufacturing Wires Filled with Powders" disclosed in Japanese Provisional Patent Publication No. 207598 of 1991 feeds a Granulated powder of substantially nonmagnetic substances into the almost closed tube to prevent the stirring of ferromagnetic ingredients to the edges of the tube to be welded. Though the latter two inventions brought about some improvement, unnegligible quantities of powders have continued to adhere to the edges of almost closed tubes to be welded.
Despite the improvements brought about by the above inventions, cracks have continued to occur in the tube diameter reduction operations, considerably impairing the product yield. Once formed, even minute cracks develop along the length of tubes as their diameter becomes smaller, which eventually grown into unnegligible lengths on finished tubes.